Biodegradation of hexachlorocyclohexane isomers in soil and food environment

Persistence of chlorinated hydrocarbon insecticides in the environment is well documented. One early introduced insecticide, hexachlorocyclohexane (HCH), popularly called BHC, was used in large quantities all over the world until recently. In India, even today, technical grade HCH is being used extensively. Theoretically, HCH has eight possible stereoisomers of which four (alpha, beta, gamma, and delta) predominate in the technical product. These isomers significantly differ between themselves with respect to their persistence and toxicity toward insects, birds, mammals, and other nontarget organisms. The relative proportion of HCH isomers is, therefore, crucial from a toxicology standpoint. This problem assumes importance in light of reports that the HCH isomers undergo interconversion in soil, water, animals, plants, insects, etc. The persistence of HCH can be attributed in part to the interconversion of HCH isomers, which also restrict their solubility. In the present review, biotransformation of HCH isomers, both under aerobic and anaerobic conditions, and their degradation pathways have been described. In addition, emphasis is given to the interconversion of HCH isomers, including interconversion mechanisms, as this area has not received adequate coverage in earlier reviews on HCH.